Accident analysis of lead-bismuth cooled fast reactor and SCO2 Brayton cycle system based on Modelica language

In the integration of Lead-Cooled Fast Reactors (LFR) and supercritical CO2 (SCO2) Brayton cycles, multi-physics coupling between the reactor core and the power conversion system governs the transient safety and operational stability. This work develops a high-fidelity dynamic model of the coupled LFR-SCO2 system via Modelica. Based on global sensitivity analysis identifying external reactivity and loop flow rates as dominant parameters, transient responses under Loss of Flow Accident (LOFA) and Unprotected Transient Overpower (UTOP) are examined. The system exhibits robust inherent safety via negative reactivity feedback (primarily Doppler effect and fuel thermal expansion). During a severe 400 pcm/2s UTOP, peak fuel centerline and cladding temperatures reach 1921.40 K and 1116.57 K, respectively, both within structural limits. System-wide uncertainty quantification, considering variations in fuel thermal conductivity and heat transfer correlations for both primary and secondary loops, indicates that the peak fuel temperature is limited to 2174.20 K under pessimistic conditions. In the SCO2 loop, UTOP-induced temperature rise boosts cycle efficiency by ∼20% via expanded enthalpy drop but causes a 344.02 K turbine inlet temperature surge. Under a -20% primary flow step (LOFA), lead-bismuth eutectic (LBE) coolant acts as a thermal buffer, keeping secondary loop pressure fluctuations <0.1 MPa and SCO2 supercritical. These results clarify cross-loop thermal coupling and provide critical quantitative data for safety design and control logic optimization of nuclear-driven SCO2 systems.